Currently, there is a tremendous interest in treating a subject having a clinical condition associated with impairment in electrical stimulation of cells or tissues. It should be appreciated that as used herein, in general, the term “a clinical condition associated with impairment in electrical stimulation” refers to a manifestation of a clinical condition due to improperly functioning neurons or retina. Such improper function can be due to impaired transmission of a signal from one cell to another cell (e.g., typically to an adjacent cell), lack of cell function even in the presence of a properly transmitted signal from another cell, or due to cell death or disrupted pathways.
Many prostheses have been developed to treat spinal cord injury by implanting an artificial electric stimulation device. Exemplary clinical conditions associated with impairment in electrical stimulation of cells and/or tissues include, but are not limited to, neural cell damage or impaired neural cell function such as retinal damage (such as retinitis pigmentosa, retinal detachment, diabetic retinopathy, and macular degeneration), optic neuropathy, glaucoma, stroke, spinal cord injury, peripheral nerve injury, demyelinating disease (such as multiple sclerosis), and central nervous system injury secondary to ischemia, compression, nerve injury, infection affecting nerve cell function, elevated intracranial pressure, elevated intraocular pressure (ocular hypertension), congenital and hereditary genetic diseases associated with impaired nerve cell function, toxic neuropathy and encephalopathy, neurological sequelae of systemic diseases such as chronic arterial hypertension, diabetes, HIV infection, systemic lupus, coagulation disorders, Parkinson's disease, Alzheimer's disease, prion disease, and paralysis. Retinal damage or impaired retinal function can lead to diminished sight and blindness. And, as the age of the general population increases, the number of people suffering from diminished sight due to these causes increases.
Functions of some cells can be replaced or approximated by providing an appropriate electrical stimulation to cells they are connected to. It should be appreciated that as used herein, a clinical condition associated with electrical stimulation impairment refers to any clinical condition that manifests itself due to an improper or missing signaling of one cell to another cell, often an adjacent cell. Several devices have been developed to attempt to restore vision loss due to retinal damage. For example, photovoltaic devices, which are attached to a portion of a retina, have been developed to replace the signals that normally emerge from rods and/or cones in a healthy eye within the retina by stimulating functioning cells. Although such devices may provide some stimulation, the devices suffer from several drawbacks.
Vision impairment can be caused by numerous factors. While many vision impairments can be corrected by corrective eyewear and surgery, not all vision impairment can be treated by such relatively simple methods. For example, some vision impairments involve problems with the light-processing functions of the eye. These problems are usually caused by abnormalities of the retina and macula such as retinitis pigmentosa and age-related macular degeneration. Vision impairments due to these causes cannot be addressed with corrective eyewear or eye surgery. It is estimated that globally over one and a half million people have progressive vision loss as a result of retinitis pigmentosa, the primary cause of inherited blindness.
To address such vision impairment, research on retinal implants has been ongoing for about two decades. One area of such research is to restore a small part of vision to people suffering from blindness due to retinitis pigmentosa or due to age related macula degeneration. One of the conventionally available devices for treating vision impairment is a chip with an array of electrodes that is placed into an epi-retinal, sub-retinal, or supra-choroidal position. With this device, electric currents emerging from the electrodes are seen by the blind person as small phosphenes. Thus, electrodes serve as pixels for presenting an image. Unfortunately, despite some encouraging results the goal of presenting a gray-scaled picture with a thousand or more pixels has not yet been reached. Other devices that have been developed include optic nerve implants, lateral geniculate nucleus implants, cortical implants, as well as non-invasive remedies such as electric tongue stimulators, and tactile stimulators.
Problems associated with currently available retinal implants include use of too simple time profiles of the electric stimulation signals. Currently, the most common time profile is the monophasic rectangular voltage pulse, which yields a biphasic current pulse. Or the biphasic rectangular voltage pulse, which yields a triphasic current pulse. Such pulses lead to the depolarization (or hyperpolarization) of a cell membrane in the first phase of the current, and to a polarization of opposite sign in the next phase of the current.
Another shortcoming in many conventional retinal implants is using only one electrode per pixel. Moreover, many research groups are using a common remote counter electrode that is far away from the electrode array. While one group in Australia is using six (6) counter electrodes around a center electrode on a hexagonal grid and, by current splitting, this group also uses a common remote counter electrode, i.e., far away from the electrode array. See Lovell et al., Engineering in Medicine and Biology Society, 2005, 27th Annual International Conference of the IEEE—EMBS, 2005, pp. 5242-5245, 17-18 Jan. 2006; doi:10.1109/IEMBS.2005.1615661]. One of the key problems associated with using counter electrodes at infinity, is that it leads to cross-talk. See, for example, Schmid et al., Electric Stimulation of the Retina; 2010, arXiv:1012.5958v1 [q-bio.NC].
Yet another problem associated with conventional retinal implants involves too little effort for shaping the electric field (or current). Simultaneous firing of neighboring electrodes leads to bunching of field lines, i.e., increased density of field lines above the electrodes. But it also means undesired cross-talk between neighboring electrodes.
Therefore, there is a need for a new apparatus and method for treating a clinical condition associated with an improper electrical signaling of a cell and/or tissue.